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Probably a 3.73 Non-LS open diff. It wasn't listed on the decoders under the F150. But I found a note that a 26 is found on the 250. So I'm only guessing.
Crawl under the truck and look at the metal tag bolted to the diff. The first 3 is probably drilled through for the bolt. But you should be able then to see the 73. Other possibilities are most likely a 55 for 3.55 gears. I'd be surprised if you had something other than 3.73 or 3.55.